The present invention is directed to a furnace for providing heated circulation air to an interior comfort space, and is more particularly directed to a gas fired forced air furnace that can be operated at a full rate or high-heat mode and at a partial rate or low-heat mode.
Conventional forced air furnaces cycle on and off to maintain a desired temperature within a comfort space i.e., within a building interior.
A thermostat senses the temperature in the comfort zone relative to a predetermined set point temperature. When the temperature is below the set point, the thermostat closes to supply thermostat ac power to the furnace as a call for heat. This causes the furnace to come on, initiating an inducer motor to flow combustion air, after which a gas valve is actuated to supply gas to the gas burners. An ignition device is also actuated to light the burners. A flame sensor then proves burner ignition and sends power to a burner delay timer. Then after a predetermined blower on delay time, which varies with furnace design, the furnace blower is actuated. The blower moves circulating room air from a return air duct through the furnace heat exchanger to pick up heat from the heated combustion products (carbon dioxide and water vapor) from the gas burners. The heated circulation air then goes into a hot air plenum and is distributed through hot air ductwork back to the comfort space. When the comfort space air is warmed sufficient to reach the thermostat set point, the thermostat terminates the call for heat. When this happens the blower and burners go through a shut off sequence and the furnace awaits the next call for heat.
The purpose of the blower time delay is to give the burners and the heat exchanger sufficient warm up time before blower actuation. This ensures that the furnace does not blow recirculating cold air back into the comfort space after a call for heat. Also, this blower-on delay allows the heat exchanger and vent gas temperatures to rise to optimal operating levels before return air is circulated. This limits the amount of condensation in the heat exchanger and in the vent or exhaust pipe during the early phase of a heating cycle.
In a modern two-stage furnace, the gas burners can be actuated at a full flow rate or high-fire mode or at a reduced rate or low fire mode, depending on the heating requirement for the comfort space. In cold weather when the heating load is great, high fire is selected, and in moderate weather when a lesser heating load is imposed low fire is selected. This can be done internally by the furnace control circuitry, based, for example, on the cycle time for the previous heating cycle.
In a conventional scheme for a two stage furnace the same blower on delay time is used for both the high fire and low fire modes. This can unduly lengthen the warm up of the heat exchanger and exhaust vent, and an unacceptable amount of vent condensation can result when the furnace is operated at low fire.